Not applicable.
The present invention is directed to polymer and solvent compositions and methods pertaining thereto that provide for the formation of a highly table porous polymer network prior to phase separation and for electroosmotic exchange of the polymerization solvent, thereby eliminating the need for high pressure purging.
Monolithic polymeric materials composed of polymerized monomers (styrenes, acrylates, methacrylates, etc.) have proven useful as the stationary phase for various chromatographic applications and particularly for applications involving miniaturized or capillary columns where traditional methods of column packing have proven to be ineffective. Thus, porous stationary phase materials that are xe2x80x9ccast-in-placexe2x80x9d or xe2x80x9ccast-to-shape by polymerization of mixtures of monomers directly within the confines of a chromatographic column, such as those disclosed in U.S. Pat. No. 5,728,457 entitled xe2x80x9cPorous Polymer Material with Gradientsxe2x80x9d and issued to Frechet et al. on Mar. 7, 1998, have been developed to address this problem. By careful control of polymerization rate, time, and temperature Frechet has produced a single molded polymer monolith that possesses desirable hydrodynamic properties by virtue of being traversed by large channels and permeated by small pores. Several variations have already been successfully used in the separation of polyaromatic hydrocarbons (PAH), PTH-labeled amino acids, peptides, and explosives.
In phase-separation polymerization, a solution of monomers is polymerized. When the polymer molecules grow sufficiently large, they separate from the inert solvent (phase separate). A liquid-liquid or liquid-solid phase separation can occur with partitioning of the unreacted monomers. If a three-dimensional network is formed before precipitation, a polymer monolith consisting of a three-dimensional network of solid polymer and an interconnected network of solvent filled pores will be formed. The structure and dimensions of the interconnected porous polymer network can be determined by controlling the proportions of solvent as well as the monomer and solvent composition.
Prior to using a polymer monolith as a chromatography separation medium or as the dielectric medium for electrokinetic pumping applications it is generally necessary to remove the polymerization solvent. Prior art processes have required the use of high pressure purging schemes to remove the polymerization solvent. Attempts to employ electric field induced flow, such as electroosmotic flow (EOF) have been unsatisfactory because prior art phase separated polymer monoliths have been cast in solvents, such as water, that because of their low conductivity do not support EOF that is large enough to purge the solvent from the polymer monolith within a reasonable period of time. Consequently, it has been necessary to laboriously purge the polymerization solvent by application of pressure to force a fluid, such as a running buffer, through the porous network. However, as the size of capillary channels is reduced and/or the cell size of the polymer stationary phase material decreases this option becomes untenable. Pressure cannot be used to purge small cell size polymer monoliths because the pressures needed for practical flow rates are typically higher than the bulk moduli of the polymer media. Thus, attempts at pressure purging small cell size polymer monoliths can result in failure of the bulk matrix or insufficient flow to exchange out the polymerization solvent in a reasonable time. Thus, it would be desirable, and in fact necessary, as capillary dimensions and cell sizes become smaller, to remove residual polymerization solvent by means other than by the use of high pressure purging.
Recognizing the advantage of being able to purge solvent from the polymer material by the use of EOF, Palm et al., Anal. Chem., 69, 4499-4507, 1997, have described a one-step process for in situ preparation of macroporous polyacrylamide gel matrices for capillary electrochromatography that can be purged by EOF. While the solvent can be purged from these formulations by the use of EOF, the gel matrices have limited stability in useful chromatographic solvents such as acetonitrile; being stable only up to about 50% acetonitrile. Moreover, polyacrylamide gels are highly swelled gels of low polymer content that rely on the swelling solvent for their structure. Thus, these gels suffer from the drawback that they cannot be dehydrated without losing their structure.
It is possible to increase the ionic conductivity of an aqueous polymerization solvent by adding salts, etc., however, to do so can change the solubility of the monomers used as well as the nature and structure of the polymer phase formed upon phase separation. Further, in order for the polymer monolith to support electric field induced EOF it is necessary to incorporate a small amount of charged monomers in the formulation. What is needed is not only a polymerization solvent that possesses sufficient ionic conductivity that it can be removed by EOF within a reasonable period of time but also a monomer or combination of monomers that will polymerize in the presence of this polymerization solvent to form a three-dimensional polymer network prior to phase separation. Further, it is necessary that this three-dimensional polymer network provide sufficient surface charge density to support EOF.
As discussed above, applications for porous monolithic polymer materials range from the stationary phase in various chromatographic applications such as electrochromatography to providing the dielectric medium for electrokinetic pumping. These applications can employ a wide range of organic as well as inorganic solutions as the mobile phase. The composition of the mobile phase that can range, by way of example, from 100% acetonitrile to aqueous solutions having a pH anywhere in the range of 2-12, and mixtures thereof. Thus, in addition to being able to support EOF, it is necessary that the polymer monolith possess a high degree of stability, which is defined as resistance to swelling, dissolution and/or structure change in a wide range of mobile phase solutions.
The present invention is directed to polymer and solvent compositions and methods pertaining thereto that provide for the formation of a highly stable porous polymer network prior to phase separation and for electroosmotic exchange of the polymerization solvent following the step of polymerization, thereby eliminating the need for high pressure purging. Because they can be rapidly cured from low-viscosity acrylate monomer solutions under UV radiation (typically in less than 30 minutes) the porous polymer monoliths of the invention provide for easy manufacturability and for ease of placement into microchannels. Further, because they readily support electroosmotic flow (EOF), the polymerization solvents can be easily exchanged for a chromatography mobile phase without the need for pressurized flow. Moreover, because they are covalently bound to a substrate, the porous polymer monoliths can withstand high pressures without being extruded from the substrate.
The monomer mixtures of the invention contain a relatively high proportion of a crosslinking material (generally≈30 vol %). Extensive crosslinking allows the porous polymer monoliths to achieve high molecular weights and, in contrast to prior art porous polymer materials imparts a high structural stability such that the polymer monolith resists swelling and/or dissolution in the presence of a wide variety of solvents.
Thus, it is an object of this invention to provide a porous polymer material that can be purged of polymerization solvent by the use of EOF.
It is another object of the invention to provide a class of polymerization solvents that posses sufficient conductivity that they can be removed from the porous polymer matrix by EOF.
It is a further object of the invention to provide a polymerization process that operates to produce a porous polymer matrix in the presence of conductive polymerization solvents.
Yet another object of the invention is to provide a stable porous polymer matrix that resists swelling and/or dissolution in a wide range of organic and aqueous liquids.
These and other objects of this invention will become apparent from the description below and accompanying drawings.